Shape-memory effect has been described for different material classes: polymers, metals and ceramics. First materials with shape-memory were metallic alloys. Shape-memory materials are stimuli-responsive materials and are used in different fields of industry, e.g in aircraft, piping, textiles, packaging, optometry and in medicine e.g. in orthopedics and minimally invasive surgery. Over shape-memory metallic alloys and ceramics, shape memory polymers have, for example, the advantage of light weight, low cost, good processability, high shape deformability and shape recoverability.
Shape-memory polymers are a class of smart and functional polymers. Shape-memory polymeric material may be biodegradable or non-biodegradable. Combination of shape-memory capability, biocompatibility, biodegradability and tailored mechanical properties can be mentioned to highlight the versatility of shape-memory polymers as biomaterials and their applicability in medical devices.
Shape-memory is a material property where the deformed polymeric material has an ability to return from a deformed, temporary shape, towards the original, permanent shape. Shape-memory polymers and products made of them can change their shapes from a temporary shape to their original shapes under appropriate activation or external stimulus such as e.g. temperature, light, pH, solvent composition, specific ions or enzymes. A change in shape induced by a change in temperature is called thermally induced shape memory effect.
Conventional shape-memory effect results from the polymer's structure like a multiblock copolymer structure. Shape memory polymers generally contain two separate phases (like different components on molecular level): a fixing phase and a reversible phase. Fixing phase may contain physical or chemical cross-linking molecular structure (cross-linked polymer network) or crystalline phase and reversible phase may be amorphous.
Shape-memory polymers have potential applications also in medical devices. If the medical device is not intended to be permanent it is possible to use biodegradable polymer(s). Promising fields are minimally invasive surgery and also scaffolding and suturing devices for assisting in tissue repair. Following publications describe the aforementioned and other applications and devices, e.g. Lendlein and Langer, Science 296 (2002) 1673-6, U.S. Pat. No. 6,281,261, EPO Pat no. 1,056,487.
One limit of prior art biodegradable shape-memory polymers is that shape-memory effect is typically thermally induced, which usually means temperatures above polymer glass transition temperature, Tg, typically temperatures between 45° to 70° C. Another disadvantage of prior art shape-memory polymers, is their low mechanical strength.
In order to improve the material properties or obtain new functions of shape-memory polymers, shape-memory composites and blends can be prepared, as described in Zheng et al., Biomaterials 27 (2006) 4288-4295, Ohki et al., Composites: Part A 35 (2004) 1065-1073. However the shape memory effect of these composites is also thermally induced, which may restrict their use in medical applications.
Biodegradable medical devices, which have shape-memory effect induced by physiological conditions at temperature of 37° C., are described in U.S Patent application 20090149856.
Some objects of the present invention are to produce composites or devices with an adequate mechanical properties and whose shape transformation at physiological conditions can be controlled to obtain rapid and increased degree of the shape transformation. This may improve e.g. an initial and short term self-locking and fixation strength of the medical devices.